Connection control in a communication system

ABSTRACT

The present invention relates to a method of controlling at least one transmission parameter of a connection between a transmitting station (BS) and receiving station (MS 0 . The method comprises the steps of receiving at the receiving station a transmission signal from the transmitting station, determining from the received transmission signal whether there exists a power up requirement or a power down requirement, and monitoring the distribution of the power up and power down requirements over a period. If a predefined form of the distribution is detected, the quality target of the connection is changed. The present invention relates also to an arrangement and a receiving station for implementing the method.

FIELD OF THE INTENTION

The present invention relates to a method of controlling at least onetransmission parameter of a connection between a transmitting stationand a receiving station in a communication system. The invention relatesfurther to an arrangement in a communication system and to a receivingstation for use in a communication system.

BACKGROUND OF THE INVENTION

In a mobile telecommunication system, such as a CDMA (Code DivisionMultiple Access) or WCDMA (Wide-band CDMA) or TDMA (Time divisionMultiple Access) system, transmission power levels between a basestation (BS) and a mobile station (MS) associated with said BS can becontinuously adjusted during an ongoing connection between the BS andthe MS. This is done in order to provide a sufficient quality for therequired transmission power levels as low as possible at the same time.By means of this it is possible to avoid “wasting” any network resourcesand MS battery resources, and to enable as great a number of mobilestations as possible to communicate simultaneously with the same BShaving only limited power resources.

One system of power control is based on Power Control (PC) commandstransmitted from one station to another to cause the other station toalter its power. The commands can be transmitted e.g. in a WCDMA closedloop. The closed loop power control mechanism between the BS and MS isused for equalising the power of signals from the MS at the BS input andalso for compensating fast power deviations from the nominal level.These closed loop PC (CL PC) commands can be sent both in the uplink(towards the base station) and in the downlink (towards the mobilestation), whereafter the BS or the MS will process the received commandand reduce/increase its transmission power towards the receiving station(i.e. MS or BS respectively) accordingly.

For example, in the currently proposed WCDMA system it is envisaged thatan outer loop PC generated by a radio network controller (RNC) of theWCDMA system will attempt to set the connection quality target (that theclosed loop follows) of a physical connection between the BS and MS tobe such that the required FER (Frame Error Ratio) target of theconnection is met with a minimal connection quality target. Theconnection quality target can be announced e.g. by means of a so calledEb/No (Signalling Energy/Noise) target or SIR (signal to InterferenceRatio) target or a similar parameter indicating some quality measurementfor the connection. The relationship is such that the connection qualitytarget (e.g. the SIR target) has to be set such that the FER remains atan appropriate level. The actual connection quality value (e.g. SIR) isthen adjusted in accordance with the target value, and should follow anychanges in the target value. The idea behind this is that by increasingthe connection quality target value the connection quality will increaseand the FER will improve.

However, if the FER target cannot be met due to e.g. a limitation in theavailable transmission power when severe interference or attenuation ispredicted, the connection quality target will start increasing eventhough this rise in the connection quality target will not help incausing a better connection between the MS and the BS. If the powerlimitation is caused by a temporary lack of power caused by a conditionsuch as slow fading or a temporarily weak connection (if, for instance,the MS is situated temporarily in a tunnel or cellar), the qualitytarget will be unnecessarily high once this condition has been removed.This will result in an excessively high transmitted power until thequality target has returned to its normal (appropriate) level. At the BSside this unnecessarily used power resource could be used fortransmission towards other mobile stations. At the MS side this willlead among other things, to unnecessary high power consumption and to apossible disturbance to other radio and/or electronic devices.

To give a more precise example, if the BS runs out of power (i.e. apower limitation situation occurs), then the mobile station MS willexperience a higher FER than the set FER target. This will result (ifnot limited by some means) in an unlimited rise of the SIR target value.In accordance with one exemplifying possibility the average rate perframe of this rise can be given by the formula.rise_per_frame=(FER−FER_(th)) step_size

-   -   where    -   FER is the actual obtained FER,    -   FER_(th) is the FER target and    -   step_size is the step size of the outer loop algorithm

Thus, if the actual FER is 2%, the FER target is 1% and the step size is0.5 dB the SIR target will in ten seconds (1000 frames) be raised by1%*1000*0.5 dB=5 dB, which can be considered to be a substantial rise.If the higher FER has been caused by e.g. shadowing and the situationchanges suddenly the SIR target will be much too high for a while afterthis condition ends. In this specific example, the SIR target woulddecrease gradually back to its appropriate value in approximately5/0.0005=1000 frames=10 seconds.

Earlier proposals to solve this problem have been based on settingabsolute limits on the values of the quality targets. There are,however, some problems associated with this type of solution. Firstly,the set absolute limits have to be relatively loose due to thevariations in the required quality target for satisfactory quality ofthe communication. Secondly, the setting of absolute limits for the MSis problematic due to the fact that the absolute value of the qualityvalue setpoint depends heavily on the used estimation method.

SUMMARY OF THE INVENTION

The embodiments of the present invention aim to at least partiallyovercome one or several of the disadvantages of the prior art proposalsin avoiding an undesired and/nt in the air interface between thetransmitting and receiving station.

According to a first aspect, the invention provides a method ofcontrolling at least one transmission parameter of a connection betweena transmitting station and receiving station in a communication systemcomprising:

-   -   receiving at the receiving station a transmission signal from        the transmitting station;    -   determining from the received transmission signal whether there        exists a power up requirement or a power down requirement;    -   monitoring the distribution of the power up and power down        requirements over a period; and    -   in the event that a predefined form of the distribution is        detected, changing quality target for the received signal.

According to a second aspect the invention provides an arrangement forcontrolling at least one transmission parameter of a connection betweena transmitting station and a receiving station in a communication systemcomprising:

-   -   a control unit for determining a power up requirement or a power        down requirement;    -   means for monitoring the distribution of the power up and power        down requirements over a period of time; and    -   means for changing a quality target of the transmission in the        event that the means for monitoring detect a predefined form of        distribution in the monitored distribution.

According to a further aspect, the invention provides a receivingstation for use in a communication system, comprising:

-   -   means for receiving a signal from a transmitting station;    -   a control unit for determining a power up requirement or a power        down requirement;    -   means from monitoring the distribution of the power up and power        down requirements over a period; and    -   means for generating and transmitting a request for transmission        parameter change to the transmitting station in the event that        the means for monitoring detect a predefined form of        distribution in the monitored distribution.

In a more specific embodiment a transmission power level parameter isalso changed. A still more specific embodiment comprises transmittingpower control commands between the transmitting station and thereceiving station, said power control commands including either thepower up or the power down request in accordance with the determinedrequirement, wherein the step of monitoring the distribution of thepower up and the power down requirements comprises monitoring therequests derived from the power control commands. According to onealternative the form of the distribution of the power up and the powerdown requirements is defined on basis of variations in the SignalInterference Ratio (SIR) target. The transmitting station can be a basestation and the receiving station a mobile station, or then vice versa.Said determining of the power up requirement or power down requirementand said monitoring of the distribution can be accomplished at thereceiving station, or then said determining of the power up requirementor power down requirement is accomplished at the receiving station andsaid monitoring of the distribution is accomplished at the transmittingstation. The step of changing the transmission parameter may comprisereturning the transmission parameter to a predefined or default value.At least some of parameters controlling the transmission parameter ofthe connection can be transmitted to the receiving and/or transmittingstation using mobile networks apparatus. In addition, it is possible touse at least two different sets of control parameters simultaneouslywhen controlling the connection.

Several advantages are obtainable by means of the embodiments of thepresent invention, as they provide a new type of solution forcontrolling the connection between transmitting and receiving stations,and for instance, for controlling the connection quality target and/orpower levels used for the transmission. By means of the proposedembodiments it is possible to prevent unnecessary high power levelsafter a temporality weak connection has returned to its normal quality.It is also possible to prevent unnecessary rise in the power level thereceiving station asks from the transmitting station in case where it isnot possible for the transmitting station to provide any more power.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention and other objects and advantages thereof will now beexplained in an exemplifying manner with reference to the annexeddrawings, in which:

FIG. 1 shows a part of a mobile communication system;

FIG. 2 shows a transmitter-receiver pair;

FIG. 3 shows a flow chart in accordance with one embodiment; and

FIGS. 4 to 6 illustrate results of simulations accomplished for theproposed method.

DETAILED DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a schematic presentation of a part of a mobile communicationsystem, disclosing a base station BS and some mobile stations MScommunicating with the BS. The MS could be moveable (e.g. a handportable mobile phone or a hand portable computer provided with a radiotransceiver facility or a communicator) or could be fixed in location(e.g. if the MS is to serve an office at a fixed side) The skilledperson is familiar with the operational principles and the variouscomponents of a mobile communication system, such as a CDMA system,WCDMA system, FDMA system or a TDMA system providing mobility for themobile station users thereof, and thus these will not be described indetail. The other parts of a functioning mobile network apparatus havealso been omitted from FIG. 1 for the reasons of clarity.

The BS transmits to each of the mobile stations MS with a power levelthat is adjusted in accordance with, for example, a Power Control (PC)command or similar message received from each of the respective mobilestations MS, that is, the transmission power levels can be different ata given moment between the base station BS and each of the respectivemobile stations MS. Correspondingly, each MS transmits towards the BSwith a power level adjusted in accordance with particular PC commandstransmitted by the BS to that precise MS. For example, in the proposedWCDMA system the PC commands would be transmitted in a WCDMA closedloop. In order to be able to accomplish this functionality, both the BSand the MS are equipped with appropriate control and processing units.

FIG. 2 shows in more detail one base station and mobile station pair.The mobile station comprises an antenna 6 via which it is arranged totransmit and receive signalling from the base station. The base stationcomprises correspondingly an antenna 16 via which it is arranged totransmit and receive signalling from the mobile station. The mobilestation is capable of transmitting a message (e.g. a closed loop powercommand) indicating that the quality of the signalling received from thebase station is too low or then that the quality is too high. At thebase station the message can be received by a transceiver unit 14 fromwhich the message is passed to a power up/down controller 13 controllingthe actual transmission power level of the transceiver unit 14. Thetransceiver unit 14 of the base station may increase the transmissionpower in order to improve the quality of the signal received at themobile station end or lessen the transmission power in order to avoidany use of unnecessarily high transmission powers in accordance withinstructions received from the unit 13.

The base station includes further a control unit 11. Control unit 11 isarranged to control the received power control commands or similarmessages and to monitor the distribution of the power up and power downrequests, as will be explained later. It is noted that even though thisexample shows the controller and the monitoring facility as a singleunit 11, they could also be in the form of separated units. It is alsoto be appreciated that the single unit could also comprise the powerup/power down control and/or any other possible control functionalitiesa transmitting station controller may have. It is noted that the MS maycomprise similar functionalities and that monitoring can also be done atthe mobile station of FIG. 2, by means of appropriate monitoring andcontrol apparatus 1 to 4 implemented in the MS.

The PC command from the MS and received at the BS may indicate that thetransmission power level toward the MS (the receiving station in thisexample) should go up (power up) or that the transmission power levelshould go down (power down). In normal operation conditions the averagedistribution between the determined power up and power down situationsshould be about 50/50 within a certain predefined period, such a 100frames or 100 seconds. If the form of the distribution within the perioddeviates from this, e.g. such that there are 80 requests for “up” andonly 20 “down”, this 80/20 distribution indicates that for some reasonthe connection does not meet the quality requirements and that thereceiving station (for instance, the control unit, such as a CPU 1, ofthe mobile station of FIG. 2) keeps on requesting more transmissionpower so as to improve the quality of the received signal. In anopposite occasion, i.e. when there are 80 requests for “down” and only20 for “up”, this form of distribution will indicate that the connectionis far better than required, and the transmission power could thus bereduced more rapidly to the normal level, i.e. to a predefinedtransmission power default level.

The need for sending a power up or a power down request is determined onthe basis of monitoring the quality of the received transmission signalat the receiving station (i.e. either at the BS or the MS). Thisdetermination can, for example, be based on monitoring whether the FER(Frame Error Ratio) meets the FER target or not. If not, the SIR (Signalto Interference Ratio) target is raised, and subsequently a power uprequest is formed and transmitted to the transmitting station in orderto improve the quality of the received transmission by increasing thetransmission power of the connection. However, if the transmittingstation cannot respond to this request, the result is that the receivingstation will still suffer from a bad quality connection, and in order tocorrect the situation it will increment the power requirement e.g. by0.5 dB. As already explained, this will only lead to an unwanted rise inthe target value, and the correction of this “unnatural” situation maytake some time.

As disclosed by the flow chart of FIG. 3, in the proposed solution thequality target (such as the Eb/No target or SIR target) is preventedfrom rising should a power limitation situation occur by returning apredefined transmission parameter of the connection, such as the powerlevel or quality target to a predefined or default value in casemonitoring of distribution of the defined power up and power down needsshows that the form of the distribution deviates a predefined amountfrom average. According to one alternative this can be accompanied bymonitoring the transmitted closed loop PC commands by the monitoringunit 11 (or unit 2 of the mobile station) in order to detect the powerup or power down requests from these commands. The monitoring may alsooccur already at the stage of determining a need for a change in thepower level at the receiving station. In any case, the logic here isthat if the transmission power is limited at the transmitting station orif the transmitting power is far too high, then the distribution for thetransmitted up/down commands will become deviated significantly from anaverage 50/50 situation in either direction (up/down) at the receivingstation, as it keeps on asking more (connection weakened) or less(connection improved) power over a certain predefined period or window.

One algorithm which can be used here is in pseudocode as follows.SIR_old=SIR_target (n)

Calculate the average amount of transmitted “up” commands during aperiod of k frames. Then

-   -   IF average>threshold1        -   SIR_target (n+k)=SIR old;    -   ELSE IF average<threshold2        -   SIR_target (n+k)=SIR_old;    -   END

The threshold values can be set in accordance with predefined controlparameters to achieve satisfactory performance. According to onepossibility, the control parameters used in the algorithms can be sentto each BS of the system over an Iub interface and/or over the airinterface from the BS to the MS. The control parameters can also becentrally updated e.g. by the network operator, e.g. in the case thatmore/less transmission power resources become available, eithertemporarily or permanently. Instead of having the control unit withinthe receiving station, the control unit for this can also be situated inanother network or there could be separate control units interfacing thenetwork including the receiving and the transmitting stations.

As can be seen from the above algorithm, when the control unit of thereceiving station determines that the form of the distribution deviatesmore than is allowed from the average distribution, it will immediatelyreturn the SIR_target to the predefined SIR_old value, whereafter theoperation will continue from this default value, and thereby excessivelyhigh target values are avoided in case of limited transmission power andthe power level is returned rapidly into a minimal appropriate levelshould the connection conditions suddenly improve. Even though thequality of the connection does not become better as such by means ofthis proposal, it does help in removing problems relating to anexcessive increase of the target value.

According to one embodiment, if the above algorithm determines the SIRtarget increase at the BS, this action shall be reported to the radionetwork controller designated by 12 in FIG. 2 which may then proceedaccordingly, e.g. reserve more power resources for that precisetransmission or send an appropriate message to the network operatorindicating that there are some problems in the power levels or othertransmission parameters.

It should be noted, that this type of algorithm can also be used in aconcatenated form, i.e. two or more different sets of control parameterscan run in parallel. In practice this can be implemented e.g. such thatthere are two monitoring periods, a shorter one and a longer one,wherein the arrangement is such that in the shorter monitoring periodthe control parameters are set such that a greater deviation in thedistribution is allowed, while the longer period averaging a greateramount of frames allows a smaller amount of deviation in thedistribution. By means of using several sets of control parameters it ispossible to improve further the system's sensitivity for different typesof variations and/or disturbances in the connection.

At present the proposed solution as such is believed to be preferablyapplicable at the MS end, considering current implementation of networkfunctionalities. However, the solution can be equally implemented at theBS side as well or instead without departing from the scope of the idea.In addition, even though the preferred implementation at the moment issuch that the determining of the need for power up or power downrequirements and the monitoring of the distribution thereof are bothaccomplished at the receiving station, this can also be implemented suchthat only said determining step of the need for power up or power downis accomplished at the receiving station and said monitoring of thedistribution is then accomplished at the transmitting station subsequentto having received the power up/power down commands or similarindication of the changed power requirements. In the latter alternativethe transmitting station can then, for example, purely ignore the powerup requests without any further processing after having detected adeviation in the distribution exceeding a threshold value, orimmediately drop the transmitting power in case a power down biaseddistribution is recognised.

In addition to the deviation of the average, the monitored form of thedistribution can also be, for instance, a certain pattern of the powerup and power down requirements indicating some special air interfacecondition. After having detected a predefined form of subsequent powerup and power down requirements, the system may change the predefinedtransmission parameter, such that the quality target or the power levelin accordance with predefined parameter values, such as return thequality target or power level to a default (lower) value or to increasethe target or power level by more than one “normal” step at once or then“freeze” the parameter to a certain value for some time. This type ofdistribution form detection can also form part of the concatenatedsolution whereby the transmission parameter adjustment will be basedsimultaneously both on the distribution deviation detection and on thedistribution pattern detection.

If the adjustment system is biased e.g. such that it will automaticallylower the power level or the quality target if no power up requests arereceived, the form of distribution used in the proposed solution canthen be derived from the proportion between the received power uprequests and the power down status.

FIGS. 4 to 6 show simulation results for the SIR target as function oftime obtained for the above algorithm when simulated with a COSSAPsimulator by Synopsys Inc. for three different FER values, which wereFER=0.013 (with unlimited PC dynamics), FER=0.0255 (with limited PCdynamics), and FER=0.0715 (with limited PC dynamics), respectively. (Theunlimited case assumes that there will be no power limitationswhatsoever, whereas in the limited case there is a transmission powerlimit). In the diagrams the horizontal axis defines the number of framesand the vertical axis defines the SIR target in dB.

In the simulation the PC commands were averaged on 20 frames periods(320 PC commands), and the threshold1 was set to equal 0.6. Theseparameters leave 0.2×320=64 PC command margin for the UP commands, i.e.the power can rise 64 dB during the average period without the algorithmgiving a false alarm (in case the PC commands are otherwise error free).The channel was a 2-tap channel with antenna diversity (uncorrelatedantennas) and the used channel speed was 3 km/h.

As can be seen from FIGS. 4 to 6, the algorithm is capable ofefficiently cutting the increase in the SIR target value and rapidlyreturning the power level into a predefined initial value. This can beconcluded from the fact that the SIR target will not become raisedpermanently even in FIG. 6 instance where the FER value is substantiallyhigh.

Thus the invention provides a clear advantage over the prior artproposals, as it enables more rapid and dynamic response to the changedtransmission conditions and makes it possible to avoid unwantedincreases in the connection quality target values in cases where it isnot possible to receive any more transmission power.

It is noted herein that while the above describes some embodiments ofthe present invention there are several variations and modificationswhich may be made to the disclosed solution without departing from thespirit and scope of the present invention as defined in the appendedclaims.

1. A method of controlling at least one transmission parameter of aconnection between a transmitting station and receiving station in acommunication system comprising: receiving at the receiving station atransmission signal from the transmitting station; determining from thereceived transmission signal whether there exists a power up requirementor a power down requirement; monitoring the distribution of the power upand power down requirements over a period of time; and in the event thata predefined form of the distribution is detected, changing a qualitytarget for the received signal.
 2. A method in accordance with claim 1,comprising changing the power level of the transmission.
 3. A method inaccordance with claim 1, further comprising; transmitting power controlcommands between the transmitting station and the receiving station,said power control commands including either the power up or the powerdown request in accordance with the determined requirement, wherein thestep of monitoring the distribution of the power up and the power downrequirements comprises monitoring the requests derived from the powercontrol commands.
 4. A method in accordance with claim 1, wherein theform of the distribution of the power up and the power down requirementsis defined on basis of variations in a Signal Interference Ratio (SIR)target.
 5. A method in accordance with claim 1, wherein the transmittingstation is a base station of a mobile communication system and thereceiving station is a mobile station.
 6. A method in accordance withclaim 1, wherein said determining of the power up requirement or powerdown requirement and said monitoring of the distribution areaccomplished at the receiving station.
 7. A method in accordance withclaim 1, wherein said determining of the power up requirement or powerdown requirement is accomplished at the receiving station and saidmonitoring of the distribution is accomplished at the transmittingstation.
 8. A method in accordance with claim 1, wherein the step ofchanging the transmission parameter of the connection comprisesreturning the transmission parameter of the connection to a predefinedvalue.
 9. A method in accordance with claim 1, wherein at least some ofcontrol parameters used for controlling the transmission parameter ofthe connection are transmitted to the receiving and/or transmittingstation using radio network apparatus.
 10. A method in accordance withclaim 9, wherein the control parameters are defined in and/or controlparameter updates are transmitted from a separate control unit.
 11. Amethod in accordance with claim 1, comprising simultaneous use of atleast two different sets of control parameters used for controlling theconnection.
 12. An arrangement for controlling at least one transmissionparameter of a connection between a transmitting station and a receivingstation in a communication system comprising: a control unit fordetermining a power up requirement or a power down requirement from asignal transmitted from the transmitting station; means for monitoringthe distribution of the power up and power down requirements over aperiod of time; and means for changing a quality target of thetransmission in the event that the means for monitoring detect apredefined form of distribution in the monitored distribution.
 13. Anarrangement in accordance with claim 12, comprising means for changingthe power level of the transmission.
 14. An arrangement in accordancewith claim 12, wherein the means for changing the transmission parameterof the connection are arranged to return the transmission parameter to apredefined value.
 15. An arrangement in accordance with claim 12,wherein the receiving station comprises the control unit, the means formonitoring distribution of the power up and the power down requirementsand the means for changing the transmission parameter.
 16. Anarrangement in accordance with claim 12, wherein the transmittingstation is a base station and the receiving station is a mobile station.17. A receiving station for use in a communication system, comprising:means for receiving a signal from a transmitting station; a control unitfor determining a power up requirement or a power down requirement;means for monitoring the distribution of the power up and power downrequirements over a period of time; and means for generating andtransmitting a request for a change in a quality target to thetransmitting station in the event that the means for monitoring detect apredefined form of distribution in the monitored distribution.